Magnetron



E. G. LINDER .Oct. 21, 1952 MAGNETRON Filed Dec. 14, 1946 3nnento Ermzrz G. Lzzzder Gttomeg Patented Oct. 21, 1952 MAGNETR-ON Ernest G. Linder, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 14, 1946, Serial No. 716,207

13 Claims. 1

This invention relates to electron discharge devices and more particularly to devices of the type utilizing a magnetic field for constrainin the motion of the electrons.

Electron discharge devices of the above type are generally known as magnetrons and find wide application in generating electrical energy at extremely high frequencies. Magnetrons in various forms are built up essentially of an electronemitting cathode electrode placed in a particular cooperative relation with an anode electrode in an evacuated envelope. A magnetic field is provided usually by a permanent magnet or an electromagnet located outside of the envelope and so oriented that the magnetic field is perpendicular to the direction of motion of the electrons emitted from the cathode.

Among the various forms of magnetrons, the more commonly used in present practice is the multi-cavity travelling wave type. While this invention is applicable to all types, for the purpose of illustration, the multi-cavity type is chosen as an example. In a multi-cavity magnetron assembly the anode electrode usually takes the form of a solid metallic block in which the cathode electrode is centrally disposed along the length of the block. The latter is divided into a number of cavity type resonators extending radially from the cathode. The physical shape and size of the resonators determine the frequency of the high frequency current generated. Since the electrons emitted from the cathode must sweep past the resonators in the interaction space between the cathode and the anode, a circular electron orbit or path must be maintained. This path is primarily determined by the velocity of the electrons and the intensity of the magnetic field. The latter must have a definite range of values to assure proper operation and a field intensity below or above this range will cause inoperativeness of the magnetron. A magnetic field of an intensity greater than that which produces oscillation will be termed here a magnetic field of static intensity.

The present invention is particularly directed to improvements in the structure as well as in the operation of magnetrons and has for its primary object the provision of means located with in the tube envelope whereby a magnetic field may be produced.

Another object of the invention is to obtain compactness of assembly eliminating the need in certain applications of a separate magnet outside the magnetron envelope.

A further object of the invention is to provide 1 a variable magnetic field by an instrumentality inside of the tube structure Without materially altering the original size of the magnetron heretofore requiring comparatively large magnets outside of the tube.

A particular feature of the invention is that control of the operation of the magnetron may be obtained with greater economy in the expenditure of energy and in a circuit not directly associated with the high frequency portion of the system.

Another feature of the invention resides in the adaptability of modulation of the magnetron for transmitting intelligence over an ultra-high frequency carrier.

An advantage resulting from the invention is that in combining the conventional magnetron assembly with the herein proposed improvement a novel method of operation results permitting greater flexibility of control adapted particularly for pulsed operation.

Other objects, features and advantages will be apparent from the following description of the invention pointed out in particularity in the appended claims and taken in connection with the accompanying drawing in which:

Figure 1 is a perspective view partly in section of a magnetron structure in accordance with the invention;

Figure 2 is an enlarged perspective view showing the magnet coil and supports;

Figure 3 is a longitudinal perspective sectional view showing a modified coil structure;

Figure 4 is an exploded sectional view of the essential elements of Figure 1 illustrating the function of the magnet coils; and

Figure 5 is a schematic circuit diagram illustrating the connections for pulsed operation or modulation of the magnetron.

Referring to the drawing, in Figure l is seen in perspective View a magnetron discharge device of conventional type except for the modification and improvement proposed by the invention. In this view the cover of the housing is removed to show the internal structure in part and certain portions are also cut away in order to show the general assembly of parts. The housing in this type of device is a solid metallic cylinder which surrounds the cylindrical anode block 2. The housing I extends beyond the block 2 and forms with a cover, not shown here, a chamber which is part of the evacuated structure. Into this chamber open the cavity resonators 3 of the anode 2. The opposite end of the device is similarly constructed, there being formed another 3 chamber identical with the one seen in Fig. 1. The housing I is surrounded with an outer covering 4 having cooling fins 5. The housing I and end plates or covers 4a constitute the vacuum envelope of the magnetron.

The Various terminals for the internal elements are brought out through the side of the housing I Th conductors of these terminals are sealedin so as to maintain a vacuum-tight connection. Terminals 6 and 6 are connected to the cathode l which is mounted coaxially within the anode block 2. Terminal 8 connects to the output coupling element which is generally a coupling loop in one of the resonators 3. The purpose of terminals 5 and 9 will be described later inasmuch as these form part of the structural improvement and salient feature of the invention.

The anode block 2 is maintained at a relatively high positive potential with respect to the oathode (by means not shown) in order to accelerate electrons emitted by the cathode toward the anode block, as in all conventional magnetrons.

In the operation of all types of magnetrons a magnetic field must be provided and so oriented that the magnetic lines of force are perpendicular to the direction of motion of the electrons to defiect the latter in circular paths past the resonators 3. In the particular structure shown the magnetic field would be parallel to the cathode 1 since the electrons emitted therefrom travel in the direction radial to the anode block 2. Heretofore, in order to provide a magnetic field of sufficient strength, a permanent magnet was usually used outside of the magnetron envelope which was clamped between the pole pieces of the magnet. As an alternative, an electromagnet coil has been used outside the magnetron envelope and coaxial with the the anode block and cathode. Often the magnet structure was several times larger than the magnetron itself.

In accordance with the present invention the instrumentality for obtaining a magnetic field is incorporated in the envelope. This is accomplished by placing an electromagnet in the form of a winding or coil having a small number of turns in the chamber adjacent the end of the anode electrode 2. Preferred construction and placements of the coil are shown by way of example in the various figures of the drawing.

cognizance is taken of the fact that other forms and placements may well be apparent to those skilled in the art without departing from the essence of the invention.

In Figure 1 an electromagnet coil it] is mounted in the chamber adjacent the anode block 2 consisting of a single layer spiral of relatively heavy wire i i. One end of coil ii] has a terminal 9 and the centrally disposed other end is connected to a supporting rod l2 which is attached to the anode block 2. This construction can better be seen in Figure 2 which is an enlarged view showing only the essential elements. The same reference characters are used to indicate identical elements in the various figures.

Referring to Figure 2, it is seen that the housing 1 is integral with the anode block 2 and extends beyond it forming the side walls of a chamber in which is placed the coil it? made up of the spirally wound wire i i. It is to be noted that the coil l may be wound in more than one layer where space permits. Moreover, in high power work the wire used for the coil may be of tubular cross-section in order to allow circulation of cooling fiuid.

A number of wire loop supports i extend from the inner wall of the housing i and are insulated therefrom b means of ceramic buttons IS. These loops envelop the wire i i and hold itparallel with the anode block 2 for proper spacing and orientation of the magnetic field. The rod [2 is of conducting material and aside from physical support for the central portion of the coil i0 is used also for electrical connection to the anode block 2. The terminal 9 of the coil it extends through a suitable aperture in the side of the housing as also shown in Figure 1. For the purpose of a clearer view of the structure, the conventional shield for the cathode l and the heater wires are not shown in Figure 2 nor the output coupling loop. These elements have no particular bearing on the invention.

A modified form of coil structure is seen in Figure 3, which is particularly suitable for easy assembly. It provides also a more uniform magnetic field around the cathode. The electromagnet coil Illa is made up of fiat metallic ribbon wound in the form of a helix. The turns of the helix are partially embedded in the inner wall of a sleeve 13 of insulating material, for example ceramic, and the sleeve I3 is secured in the chamber adjacent one end of the anode block. One end 9a of the winding is brought out through the housing and the other end [Ed is attached to the anode block 2. A second coil Wat is embedded in a sleeve it'd of the same material as sleeve 13 secured in the chamber adjacent the opposite end of the anode block 2. One end S'a' is brought out through the housing and the other end iZ'a is attached to the anode block 2, as shown. The two coils Illa and lil'a are wound in the same direction so that they constitute a single winding in two sections.

In the above description of Figure 2 only one end view of the structure was considered. In order to visualize both ends which are similarly constructed, reference is to be had to Figure 4 which is an exploded view and a longitudinal perspective section of the magnetron including the coil structure shown in Figures 1 and 2. Here, also, elel .ents non-essential to the invention have been omittedin order to present a clear picture. It is seen that the coil if] has a counterpart in coil I0 and the two coils are connected in series by means of the supporting rods 12 and i2 and the anode block 2 to which the rods [2 and i2 are attached by brazing. The other end of coil H) has a terminal 9' shown in Figure 1.

Coils I0 and ID are connected in series in such manner that the direction of their turns continues. In other words, coils l0 and I0 may be regarded as one coil in two sections which, when energized by direct current, will produce a mag netic field and perform the same function as a conventional magnet having the pole pieces M and I4 oriented as indicated in Fig. 4. The energizing of the coil is effected by connecting a suitable current source to terminals 9 and 9 at which the outer ends of the coils l9 and i0 terminate.

In certain applications the wire H of which coils l0 and ID are wound may be of tungsten to permit operation at a very high temperature, since it is placed within th evacuated space of the tube. In this manner it is possible to ob tain, with a small coil having only a few turns easily accommodated in the vacuum tube structure, a very strong magnetic field, particularly for pulsed operation of the magnetron, which finds wide application in the electronic art. The magnet coil may be used to advantage as a means for producing a strong magnetic field. and also The following calculation shows that acoil of few' turns may be used for producing strong operating magnetic fields in a magnetron discharge device. The field intensity ina coil is given by the equation where N is the number of turns, i is the current in amperes per turn, and Z is the length in centimeters. For example, if N equals 20, 1 equals 3, and i equalslOOO, we have This is sufficient to produce one centimeter electromagnetic waves. I

If the duty cycle is that is, if the magnetron is pulsed so as to operate only of the time, then the average current is 1 ampere for a peak current of 1000 amperes. This is a low value from the standpoint of coil power dissipation, and actually considerably larger duty cycles would .be permissible.

From the above, it is seen that a magnetron device complete in all essential operating elements may be had in one hermetically. sealed structure. For certain applications, particularly in :pulsed operation, the bulky external magnet which heretofore had to be provided as a separate part in connection with magnetrons as shown in Fig. 4 may be replaced entirely by the small, simple coil structure as an integral part of the device. In some cases, however, it is desirable to retain the external magnet and use it together with the internal magnet coil or coils. In this'manner a novel method of operation results which is advantageous in several ways.

H gauss) The external magnet is preferably so chosen as to produce a magnetic field of static intensity; in other words, a magnetic field of an intensity greater than that which produces oscillations of the magnetron. The internal electromagnet coil on the other hand is used to produce a magnetic field superimposed on and in opposition to the static field of the external magnet and of sufficient strength partially to neutralize this field and thus reduce the resultant field to the oscillation value. Field strength values can easily be had in such proportions that the internal coils need to produce only a comparatively weak magnetic field to counterbalance the excessive field of the external magnet. In this manner smaller coils of a lower current strength can be used In this circuit there is also a rheostat R to adjust the current in the coils in and H! to a suitable value and a switch marked pulser which represents any suitable device closing the circuit at a selected rate. In place of the pulser, a modulator, schematically shown by a microphone may be used, the function of which will be explained later.

Considering pulsed operation a an example, the field intensity of the magnet pole pieces l4 and I4 is so chosen as to be excessive, namely, a static intensity causing inoperative condition of the magnetron. The polarity of the source, thatiis the battery I5, with respect to the coils l0 and I0 is so chosen that the magnetic field produced upon current fiow in the coils l0 and I0 opposes that of the permanent magnet. Consequently, upon energizing coils l0 and 10', the opposing field created partially neutralizes the static intensity of the field supplied by the permanent magnet and operation of the magnetron results. In this type of operation the coils Ill and I0 function primarily as control elements for triggering or actuating the magnetron. Cathode-anode current does not flow between the pulses. I 1

An additional feature-in connection with the magnet coils constructed in accordance with the invention is their use as simple modulator coils for amplitude modulation of the magnetron output. In place of the pulser, as indicated in Figure 5, a modulator indicated by the microphone I1 is used to vary the current through the coils I0 and H) at the modulation frequency rate; for example, at an audio frequency ratefor speech transmission.

It is to be noted that the coils in and I6 are connected in series through the anode block 2. In this manner the coils are at anode potential and should their high current operation require incandescence which might liberate electrons from the coils there will be no electron flow to the anode. The customary cathode shield (not shown) prevents also electron fiow from the oathode to the coils in the same manner as it prevents electron flow to the covers closing the spaces at the ends of the anode block.

What I claim as new is:

1. An electron discharge device of the magnetron type comprising an evacuated housing enclosing a cathode electrode and an anode electrode coaxially surrounding said cathode electrode, and means for producing a variable magnetic field in the interaction space between said electrodes comprising an electromagnet coil supported within said housing adjacent to and coaxial with said electrodes, said coil having a small number of turns of relatively heavy wire, whereby it is capable of carrying high peak currents, said means being adapted to produce a magnetic field in said interaction space parallel to the axis of said electrodes.

2. An electron discharge device according to claim 10, further comprising means including a magnet mounted adjacent the outside of said housing for producing a static magnetic field in said interaction space parallel to the axis of said electrodes.

3. An electron discharge device accordingto claim 10, wherein said anode electrode comprises a plurality of cavity resonators surrounding said cathode electrode. I

4. An electron discharge device according to claim 19, further'comprising means mounted adjacent said electrodes for producing a second magnetic field in said interaction space parallel tothe axis of said electrodes.

5. An electron discharge device of the magnetron type comprising an anode block electrode having a plurality of cavity resonators, a cathode electrode axial to said block, an evacuated housing for said electrodes, and means for producing a magnetic field for said electrodes comprising an electromagnet coil having two connected sections sealed in said housing and supported therein, one of said sections being on one side of said anode electrode and the other of said sections being on the other side of said anode electrode, said sections being adapted to produce a magnetic field in the direction parallel to said cathode electrode.

6. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having portions extending longitudinally therefrom, said portions enclosing a chamber at each end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chambers, a coil of conducting wire adjacent one end of said block and supported in one of said chambers, a similar coil adjacent the other end of said block and supported in the other of said chambers, a connection between said coils comprising a conducting member secured to one terminal of each of said coils and to said block, and connectors for the other terminals of said coils extending from said housing and sealed therein.

7. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having portions extending longitudinally therefrom, said portions enclosing a chamber at each end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chambers, terminals for said cathode extending from said housing and sealed therein, a coil comprising a helical winding of conducting material concentrically disposed adjacent one end of said block and supported in one of said chambers, a similar coil adjacent the other end of said block and supported in the other of said chambers, and an electrical connection between said coils.

8. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having portions extending longitudinally therefrom, said portions enclosing a chamber at each end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chambers, terminals for said cathode extending from said housing and sealed therein, a coil comprising a helical winding of a metallic ribbon concentrically disposed adjacent one end of said block in one of said chambers, a support for said winding comprising a sleeve of insulating material, said ribbon being partially embedded in the inner wall of said sleeve, the flat side thereof parallel with said block, and a similar coil adjacent the other end of said block and supported in the other of said chambers.

9. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having portions extending longitudinally therefrom, said portions enclosing a chamber at each end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chambers, terminals for said cathode extending from said housing and sealed therein, a coil comprising a spirally wound wire concentrically disposed adjacent one end of said block and supported in one of said chambers, a similar coil adjacent the other end of said block and supported in the other of said chambers, a connection between said coils comprising a conducting member secured to one terminal of each of said coils and to said block, and connectors for the other terminals of said coils extending from the side of said housing and sealed therein.

10. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having portions extending longitudinally therefrom, said portions enclosing a chamber at each end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chambers, terminals for said cathode extending from said housing and sealed therein, a coil of conducting wire wound in a layer adjacent one end of said block in one of said-chambers, a second coil adjacent the other end of said block in the other of said chambers, supports for said coils including a plurality of ceramic insulators on the inner surface of said chambers, said insulators having wire loops around turns of said coils, and a connection between said coils comprising a rod of conducting material secured to one terminal of each of said coils and to each end of said block, said rod being bent radially in said chambers forming a central support for said coils.

11. An electron discharge device of the magnetron type comprising an annular block electrode, an elongated electrode coaxially mounted in said block electrode, an evacuated housing enclosing said electrodes, one of said electrodes constituting an anode and the other constituting an electron-emissive cathode, and means for producing a magnetic field for said electrodes comprising an electromagnetic coil having two connected sections sealed in said housing and supported therein, one of said sections being on one side of said block electrode and the other of saidsections being on the other side of said block electrode, said sections being coaxial with said electrodes to produce a magnetic field extending in the direction of the axis of said coaxial electrodes.

12. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an evacuable housing surrounding said block and having a portion extending longitudinally therefrom at one end, said portion enclosing a chamber at said end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chamber, terminals for said cathode extending from said housing and sealed therein, a coil comprising a helical winding of a flat metallic ribbon concentrically disposed adjacent one end of said block in said chamber, and a support for said winding comprising a sleeve of insulating material, said ribbon being partially embedded in the inner wall of said sleeve with its major transverse axis perpendicular to said inner wall.

13. A magnetron tube structure comprising a cylindrical anode block, an elongated cathode axial thereto in the center of said block, an

evacuable housing surrounding said block and having a portion extending longitudinally therefrom at one end, said portion enclosing a chamber at said end of said block, a plurality of cavity resonators radially disposed in said block and terminating in said chamber, terminals for said cathode extending from said housing and sealed therein, a coil of conducting wire spirally wound in a layer adjacent said end of said block in said chamber, and supports for said coil including a plurality of ceramic insulators mounted on the inner surface of said chamber and having wire loops around the outermost turn of said coil and a rod of conducting material mounted on said block and secured to the inner terminal of said coil.

ERNEST G. LINDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,564,070 Huppert Dec. 1, 1925 1,666,858 Jahn Apr. 17, 1928 1,742,662 Miller Jan, 7, 1930 1,931,737 Parisi Oct. 24, 1933 2,233,779 Fritz Mar. 4, 1941 2,324,772 Gibson July 20, 1943 2,412,824 McArthur Dec. 17, 1946 2,444,435 Fisk July 6, 1948 2,450,629 Bondley Oct. 5, 1948- 

